Epoxy resin compositions have been used as semiconductor device encapsulants for over 25 years as noted by reference to U.S. Pat. No. 3,449,641, granted Jun. 10, 1969.
Anhydride-cured epoxy resin encapsulants used in flip-chip manufacturing methods are described in U.S. Pat. No. 4,999,699, granted Mar. 12, 1991, and U.S. Pat. No. 5,250,848, granted Oct. 5, 1993. Encapsulant-forming compositions are applied after electrical interconnection.
The application of an encapsulant-forming (encapsulating) composition prior to interconnection by reflow soldering, wherein electrical interconnection occurs in the presence of an encapsulating composition is described in U.S. Pat. No. 5,128,746, granted Jul. 7, 1992. In U.S. Pat. No. 5,128,746, flip-chip production methods are described where electrical interconnection is achieved by adding a fluxing agent to a mixture of epoxy resin and curing agent prior to cure. During reflow soldering, the fluxing agent is activated and the resin is cured.
The use in the prior art of cross-linking agents having flux properties is found in PCT International Publication Number WO 93/06943, published Apr. 15, 1993 and its U.S. counterpart, U.S. Pat. No. 5,376,403, granted Dec. 27, 1994. In the publication, enhanced sintering is described using a protected cross-linking agent with fluxing properties in a metal powder and epoxy resin system where the solder is used to form the conductive film. Solder powder addition is used to sinter the metal powder, typically copper or silver, before setting the resin in order to create solid electrically conductive bridges between the powdered metal particles.
The use of synthetic thermosetting polymer resins together with soldering flux agents is described in U.S. Pat. No. 3,791,027, granted Feb. 12, 1974. Therein epoxide resin compositions are described wherein fluxing agents react with the epoxide resin to strengthen solder joints.
Electrically conductive adhesive compositions in which solder powder, a chemically protected cross-linking agent with fluxing properties and a reactive monomer or polymer (inclusive of epoxy resins) are described in U.S. Pat. No. 5,376,403, granted Dec. 27, 1994.
In the present invention, the fluxing additive of U.S. Pat. No. 5,128,746 is eliminated, while the function of fluxing is retained by selecting a cross-linking agent that has the property of also operating as a fluxing agent. In the flip-chip production method of the present invention, where electrical interconnection occurs within the encapsulating composition, the preferred embodiment of the invention resides in selecting a combination of dual functioning cross-linking agent and thermosetting resin or combination of such agent and thermosetting resin with a selected catalyst and controlling the sequence of flow soldering and gel formation to avoid inhibition of soldering. This is accomplished by providing an encapsulant composition, which, at surface mount reflow profile conditions, in which the gel formation (gel point is reached) after solder melt; that is, the gel is formed/gelation occurs after reflow soldering, whereby soldering is not inhibited.
As noted by reference to “MANUFACTURING TECHNIQUES FOR SURFACE MOUNTED ASSEMBLIES,” Wassink, R. K. and Verguld, M. F., 1995 ELECTROCHEMICAL PUBLICATIONS, LTD., soldering methods (and equipment) have converged from various IR soldering concepts to one main method, namely, hot-convection soldering. Besides this method other methods are used, but only in specific cases, such as resistance soldering for outer lead bonding of TAB and for soldering on foils.
In Wassink et al., at pages 275, 276, a typical profile for reflow soldering is described. A hot-air convection soldering oven having a number of zones whose temperature can be controlled separately is used in order to attain the desired temperature profile along the length of the entire oven. Such profile enables all joint areas to reach the soldering temperatures with limited temperature differences between the joint areas of components with different thermal mass.
Wassink et al. describes the typical three step heating approach of the prior art frequently used in reflow soldering using multiple hot-air convection ovens.
As also described in Wassink et al., the three steps are:                (i) starting with rapid heating to bring heat into the product (this reduces the length of the oven);        (ii) second step concerns temperature equalizing, i.e., to reduce the temperature differentials; usually a kind of temperature plateau for the hottest parts is pursued while the temperature rise of the coldest parts is chosen to be relatively slow; the effectiveness of this step can easily be assessed by the temperature differentials that exist on the assembly just before it enters the next step;        (iii) final rapid heating and subsequent cooling.        
As further described in Wassink et al., each limit of the profile is determined by the maximum allowable thermal load of one of the parts of the assembly to be soldered.                The maximum (peak) temperature is determined by the base material of the printed board. Higher temperatures than 280° C. will cause delamination. (Note: In most cases the printed board is the hottest component.)        The minimum soldering (peak) temperature is determined by the wetting of component metallizations.        The maximum time and temperature of the equalize region is determined by the solder paste. In the case of too heavy a thermal treatment, the activator (flux) in the solder paste will be consumed already at this stage of the process.        The time for which the solder is in the molten stage (in combination with a maximum temperature) is restricted by the formation of intermetallic layers inside the soldering joint. These layers make the soldered joint more brittle.        
The specific values of the mentioned boundaries are determined based on the components and board material used.
A surface mount reflow profile for a 63 Sn/37 Pb solder illustrating the typical ranges is shown in FIG. 4.
In U.S. Pat. No. 3,791,027 (“Angelo”), the disclosure of which is incorporated herein by reference thereto, polymers and other materials which contain chemical functionalities; such as, amide, amino, carboxyl, imino, and mercaptan; which serve as flux agents are described. When soldering metals, these materials can be combined with materials which contain other functionalities; such as, epoxide and isocyanate to produce thermosetting polymers. Angelo describes three polymer categories in his invention which are set forth below.    1. Chemical functionalities; such as, carboxy terminated polybutadiene and carboxy terminated polyisobutylene, which, when used alone, do not harden and are easily removed with solvents. These are in essence fluxes and contain the same chemical functionalities found in traditional soldering fluxes.    2. Formulations that are non-crosslinking and can be softened or melted with the addition of heat. Examples cited in Angelo include Versarid 712 and Acryloidat 70. Since cross-linking does not take place, these formulations are similar to standard rosin or resin based fluxes frequently used in reflow soldering which contain chemical functionalities such as amino, carboxyl, amide, etc. Thus the same chemical functionalities are present both traditional rosin and resin fluxes and in the examples cited in Angelo which do not chemically cross-link to form thermoset polymer and hence may be removed by using a solvent or may be reheated and remelted to enable resoldering of solder joints.    3. Combinations of materials, which contain the chemical functionalities necessary to promote solder wetting; such as, carboxy, amino, etc., and materials that react chemically to form thermosetting polymers that cannot be easily removed through use of a solvent or reheated and remelted. Specifically Angelo shows examples of combinations of materials which contain such functionalities with epoxy resin materials which, when heated, form cross-linked networks which are not easily removable or cannot be remelted. Angelo cites the usefulness of such combinations to reside in their ability to reinforce the strength of the solder joint in situations when there is a low probability that a solder joint will need to be resoldered.
Pennisi, et al., U. S. Pat. No. 5,128,746, also describes the use of combinations of materials which contain chemical functionalities known to serve as fluxes and materials; such as, epoxy resins, which when reacted with the addition of heat, form chemically cross-linked polymers which add strength to solder joints and are not easily removable. Although Pennisi describes the function of the epoxy thermoset polymers as providing environmental protection to the flip-chip, the epoxy encapsulant described by Pennisi is also known to strengthen the fragile solder joints. Pennisi lists flux agents; such as, malic acid and other dicarboxylic acids that remove metal oxides and promote solder wetting. In essence, a material, malic acid, containing the carboxyl functional group, which is known to promote solder wetting, is combined with materials, epoxy resins, that form cross-linking, thermoset polymers.
In a third example, described in Capote, U.S. Pat. No. 5,376,403, a material containing a chemical functionality, such as carboxyl, known to assist in solder wetting, is combined with materials that form cross-linking thermoset polymers that are used in ink systems that assist in the fusing of low melting alloy powders with high melting metals and assist in the adhesion of the resultant metal network to a substrate.
In each case (Angelo, Pennisi, Capote) in which a material containing a chemical functionality, known to promote solder wetting, is combined with materials such as epoxy resins that form thermoset polymers, a method of heating is described in which the assembly is heated rapidly above the solder melt point. The application of temperatures above the solder melt point 183° C. is critical as the solder must liquify in order to wet the surface metal.
As thermosetting polymers are initiated by the application of heat in order to stimulate cross-linking reactions, it becomes necessary to understand the cure kinetics involved in the curing of the material combinations selected. By chemically protecting the cross-linking material of the combination, Capote ensures that the cross-linking reactions are delayed and appropriate during the rapid heating process described in his invention.
Similar heating methods are described in Angelo and Pennisi, who both describe the application of heat during the soldering process as quick and rapid. As described previously in Wassink et al., a three step heating profile is typically used to solder electronic components to substrate boards. Rapid heating, as called for in Angelo, Pennisi and Capote, would adversely affect the parts and assemblies during soldering. This includes damage to components at high thermal excursion rates.
Thus, one frequently finds the heating step to be done using multizone ovens which allow materials in assemblies to achieve thermal equilibrium at temperatures above room temperature but lower than the solder melt point (183° C.) in order to reduce thermal shock and subsequent damage. In SMT, this heating process in known as a surface mount reflow profile.
Therefore, in using combinations, as set forth by Angelo, Pennisi and Capote, in which materials containing chemical functionalities; such as carboxyl and amino, known to promote solder wetting, are combined with materials that form cross-linked thermoset polymers through the addition of the heat, heating processes are used that do not involve a rapid heating rate to the solder temperature but instead allow materials to be used in the final assembly to reach a thermal equilibrium. Above room temperature but lower than soldering temperature, it becomes critical to understand the cure kinetics of the combination of thermosetting materials, in view of the desired non-rapid heating profile in order to prevent significant crosslinking of the combination prior to solder melt point.